Analysis of unsteady mixed convection of Cu–water nanofluid in an oscillatory, lid-driven enclosure using lattice Boltzmann method

The unsteady physics of laminar mixed convection in a lid-driven enclosure filled with Cu–water nanofluid is numerically investigated. The top wall moves with constant velocity or with a temporally sinusoidal function, while the other walls are fixed. The horizontal top and bottom walls are, respectively, held at the low and high temperatures, and the vertical walls are assumed to be adiabatic. The governing equations along with the boundary conditions are solved through D2Q9 fluid flow and D2Q5 thermal lattice Boltzmann network. The effects of Richardson number and volume fractions of nanoparticles on the fluid flow and heat transfer are investigated. For the first time in the literature, the current study considers the mechanical power required for moving the top wall of the enclosure under various conditions. This reveals that the power demand increases if the enclosure is filled with a nanofluid in comparison with that with a pure fluid. Keeping a constant heat transfer rate, the required power diminishes by implementing a temporally sinusoidal velocity on the top wall rather than a constant velocity. Reducing frequency of the wall oscillation leads to heat transfer enhancement. Similarly, dropping Richardson number and raising the volume fraction of the nanoparticles enhance the heat transfer rate. Through these analyses, the present study provides a physical insight into the less investigated problem of unsteady mixed convection in enclosures with oscillatory walls

Special topic on turbulent and multiphase flows

International audienc

A pore-scale assessment of the dynamic response of forced convection in porous media to inlet flow modulations

An increasing number of technologies require prediction of unsteady forced convection in porous media when the inlet flow is unsteady. To gain further insight into this problem, the unsteady equations of continuity, Navier Stokes and energy are solved within the pores formed by several cylindrical flow obstacles. The system is modulated by sine waves superimposed on the inlet flow velocity, and the spatio-temporal responses of the flow and temperature fields are calculated. The results are then utilised to assess the linearity of the thermal response represented by the Nusselt number on the obstacles. It is shown that for linear cases, a transfer function can be devised for predicting the dynamic response of the Nusselt number. It is further argued that such a transfer function can be approximated by a classic low-pass filter which resembles the average response of the individual obstacles. This indicates that there exists a frequency threshold above which the thermal system is essentially insensitive to flow modulations. The results also show that changes in Reynolds number and porosity of the medium can push the dynamic response of the system towards non-linearity. Yet, there appears to be no monotonic change in the linearity of the response with respect to the Reynolds number and porosity. In general, it is found that for low Reynolds numbers, the dynamics of heat convection can be predicted decently by taking a transfer function approach. The findings of this study can enable further understanding of unsteady forced convection in porous media subject to time-varying inlet flows

Phase change dynamics in a cylinder containing hybrid nanofluid and phase change material subjected to a rotating inner disk

In this numerical study, the phase change dynamics of a 3D cylinder containing hybrid nanofluid and phase change material (PCM) is investigated with a finite element solver. The PCM consists of spherical encapsulated paraffin wax, and the flow is under the forced convection regime. The dynamic features of the phase change process are studied for different values of the Reynolds number (between Re=100 and 300), the rotational Reynolds number of the inner disk (Rew=0 and 300), and the size of the rotating disk (length between 0.1L and 0.55L; height between 0.001H2 and 0.4H2). The flow dynamics and separated flow regions are found to be greatly influenced by the rotational speed and size of the inner disk. As Re is increased, the difference between the transition times at different rotational disk speeds decreases. At Re=100, a 21% reduction in the phase transition time is observed when the inner disk rotates at the highest speed as compared to the motionless case. Up to a 26% variation in the phase transition time occurs when the size of the inner rotating disk is varied. A 5 input-1 output feed-forward artificial neural network is applied to achieve fast and reliable predictions of the phase change dynamics. This study shows that introducing rotational effects can have a profound effect on the phase change dynamics of a hybrid nanofluid system containing phase change material

Special Issue on Multiphase and Turbulent Flows in Energy Engineering Applications

International audienc

Breaking the symmetry of a wavy channel alters the route to chaotic flow

We numerically explore the two-dimensional, incompressible, isothermal flow through a wavy channel, with a focus on how the channel geometry affects the routes to chaos at Reynolds numbers between 150 and 1000. We find that (i) the period-doubling route arises in a symmetric channel, (ii) the Ruelle-Takens-Newhouse route arises in an asymmetric channel, and (iii) the type-II intermittency route arises in both asymmetric and semiwavy channels. We also find that the flow through the semiwavy channel evolves from a quasiperiodic torus to an unstable invariant set (chaotic saddle), before eventually settling on a period-1 limit-cycle attractor. This study reveals that laminar channel flow at elevated Reynolds numbers can exhibit a variety of nonlinear dynamics. Specifically, it highlights how breaking the symmetry of a wavy channel can not only influence the critical Reynolds number at which chaos emerges, but also diversify the types of bifurcation encountered en route to chaos itself

Introduction to Advances in Sustainable Hydrogen Energy.

Nader Karimi, Larry K. B. Li, Manosh C. Paul, Mohammad Hossein Doranehgard and Freshteh Sotoudeh introduce the RSC Advances themed issue on Advances in Sustainable Hydrogen Energy

Electro-magnetohydrodynamics hybrid nanofluid flow with gold and magnesium oxide nanoparticles through vertical parallel plates

The hybrid nanofluid flow under suspension of Gold and Magnesium oxide nanoparticles (Au/MgO-NPs) propagating between vertical parallel plates is investigated. Sodium alginate third-grade non-Newtonian fluid is used as the base fluid. The effect of electro-magnetohydrodynamics is also taken into account. The energy equation also includes the effect of Joule heating and viscous dissipation. Due to the nonlinear nature of the formulated differential equations, perturbation strategy is utilized to acquire the analytical solutions. Discussion and plotting are presented with respect to most significant parameters. It is analyzed that the rate of heat transfer is dramatically increased, and this is owing to an increase in the thermal conductivity of the fluid due to hybrid nanofluid. With increment in buoyancy convection parameter and electric field parameter, the flow is accelerated. It is also noted that the temperature is boosted with increasing nanoparticle volume fraction of both magnesium oxide and gold nanoparticles. A comparison with previously studied results is also included. The applications of the work include novel thermal duct processing technologies in biomedical, nuclear and process engineering